Digital communication systems, such as modem systems, are well known in the prior art. Such systems typically employ timing recovery techniques that are utilized to recover the symbol rate at which the data is transmitted. Such systems also use synchronization techniques that are utilized to align the receiver clock with the transmitter clock. Indeed, the prior art is replete with various timing recovery and synchronization techniques; several timing recovery schemes are discussed in Lee & Messerschmitt, DIGITAL COMMUNICATION, pp. 737-764 (2d ed. 1996), the contents of which are incorporated herein by reference.
Many modem systems transmit synchronization signals concurrently with the data transmitted from the transmit modem; the synchronization signals are detected by the receive modem and processed to synchronize the receive modem to the transmit modem. Often, the synchronization signals are transmitted near the beginning of a "handshaking" procedure, during a receiver training procedure, or periodically during data transmission (to resynchronize the receive modem with the transmit modem). In addition to the synchronization of timing recovery loops, such synchronization signals are often employed to initialize automatic gain control (AGC) circuits.
The specific synchronization technique and the particular synchronization signal used by a given communication system may depend on the design of the system itself. For example, a given synchronization signal may be configured to be easily detectable at the receiver and to effectively convey the timing phase of the transmitted signal. Thus, the precise format of the synchronization signal may vary according to the detection scheme utilized at the receiver and the characteristics of the transmission channel. Consequently, the synchronization techniques and synchronization signals used in prior art modem systems may be predetermined in accordance with internationally recognized operating standards and protocols. Indeed, such techniques may be effective when utilized in the context of standardized modem systems having wholly compatible hardware and software.
Unfortunately, such rigidly configured synchronization signals may not be desirable in the context of a pulse code modulation (PCM) modem system, such as a 56 kbps modem system, which may employ any one of a number of different timing recovery schemes. For example, a particular PCM modem system may use a timing recovery method that processes unequalized signals; such timing recovery methods typically require timing initialization prior to training of the receiver equalizers. Alternately, a PCM modem system may use a decision-based timing recovery method for which initial timing phase is unimportant for proper operation. Consequently, different timing recovery techniques may require different synchronization signals (e.g., a signal with rich spectral content including energy at the Nyquist frequency for the former method and a relatively simple, easy-to-detect signal for the latter method). In addition, different AGC schemes may require differently configured synchronization signals for acceptable performance. For example, it may be desirable to provide a synchronization signal having a wide spectrum to facilitate an accurate initialization of an AGC circuit during transmission of the synchronization signal. On the other hand, if accurate AGC initialization is not required during this time, then a simpler synchronization signal having less spectral content may be utilized.
Conventional modem systems typically operate with a 40 dB signal-to-noise ratio, which may be acceptable for transmission at relatively low speeds. In contrast, 56 kbps modem systems may require a signal-to-noise ratio of 60 dB or more. Accordingly, the AGC circuit in a 56 kbps system may require a more accurate initialization to properly fix the AGC parameters to reduce the noise contributed by the AGC circuit. In addition, an accurate timing recovery technique that utilizes an optimum synchronization signal may be desirable in order to achieve the stricter signal-to-noise requirement for 56 kbps systems.
Accordingly, the specific receiver design in a PCM modem system may dictate the particular configuration of the synchronization signal used by the system. Such synchronization signals may be short and simple tones or complex multi-tone signal patterns. However, a synchronization signal that is effective for one PCM receiver may be unsatisfactory for use with another PCM receiver; such compatibility problems may detract from the performance of these PCM systems. In addition, it may be difficult to detect or utilize prior art synchronization signals that are governed by current standards and protocols where such synchronization signals are not optimized for use with the specific receiver.